Can dehydration affect complete blood count (CBC) results, leading to a false diagnosis of anemia?

Dehydration Can Cause False Anemia on CBC Results

Yes, dehydration can cause falsely normal or elevated hemoglobin and hematocrit values on a CBC, potentially masking true anemia or making it appear less severe than it actually is. 1, 2

How Dehydration Affects CBC Results

Dehydration affects blood composition in the following ways:

• Hemoconcentration effect: When a patient is dehydrated, there is a decrease in plasma volume while the red blood cell mass remains unchanged, leading to a relative increase in hemoglobin concentration and hematocrit 1

• Spurious elevation: This creates an artificially elevated hemoglobin/hematocrit reading that does not reflect the true red blood cell mass 1

• Masking true anemia: A patient with underlying anemia may show falsely normal or even elevated hemoglobin values when dehydrated 2

Distinguishing Dilutional Anemia vs. True Anemia vs. Dehydration

Parameter	Dehydration	True Anemia	Dilutional Anemia
Hemoglobin/Hematocrit	Normal or elevated	Low	Low
Ferritin	Normal	Low (iron deficiency)	Normal or elevated
Transferrin saturation	Normal	Low (iron deficiency)	Normal or elevated
RDW	Normal (≤14%)	Elevated (>14%)	Normal
Clinical context	Signs of fluid deficit	Blood loss, poor intake	Recent fluid administration
Response to hydration	Hb/Hct decreases	No change in Hb/Hct	No change in Hb/Hct

Clinical Implications and Assessment

When to Suspect Dehydration Affecting CBC Results

• Patient shows clinical signs of dehydration (dry mucous membranes, decreased skin turgor, hypotension)
• Recent history of fluid losses (vomiting, diarrhea, excessive sweating)
• Elevated BUN/creatinine ratio
• Concentrated urine (high specific gravity)
• Normal or elevated hemoglobin despite clinical suspicion of anemia

Proper Assessment Protocol

1. Assess hydration status before interpreting CBC results
2. Repeat CBC after rehydration if dehydration is suspected
3. Consider additional iron studies (ferritin, transferrin saturation) to evaluate for true anemia
4. Evaluate red cell indices (MCV, MCH, MCHC) which may help distinguish true anemia from hemoconcentration

Common Pitfalls to Avoid

• Misdiagnosis: Failing to recognize dehydration as a cause of falsely normal hemoglobin values can lead to missed diagnosis of underlying anemia 2

• Improper sampling technique: Excessive squeezing ("milking") of the finger during capillary blood sampling can contaminate the sample with tissue fluid, leading to falsely low readings 1

• Failure to repeat testing: Not repeating CBC after rehydration can lead to incorrect assessment of a patient's true hematologic status

• Overlooking other factors: Factors like altitude, smoking, and race can also affect hemoglobin/hematocrit values and should be considered in interpretation 1

Best Practice Recommendations

1. Always assess and document hydration status when interpreting CBC results
2. In dehydrated patients, repeat CBC after adequate rehydration to obtain accurate baseline values
3. Use multiple parameters (not just hemoglobin/hematocrit) to assess for anemia
4. Consider the clinical context and patient presentation when interpreting laboratory values
5. When in doubt about the cause of abnormal CBC results, consult with a hematologist or laboratory medicine specialist

By understanding how dehydration affects CBC results, clinicians can avoid misinterpretation of hemoglobin and hematocrit values and ensure accurate diagnosis and treatment of anemia.